How can I translate the current hardcoded text with the key from the language file (messages.en.xliff)?

I tried to use the

{% trans %} translation_key{% endtrans %}

with no success. Symfony returns this error

A message must be a simple text in 'ProjectEventsBundle:Default:show_event.html.twig'

500 Internal Server Error - Twig_Error_Syntax

{% transchoice count %}
{0} The current hardcoded text|{1} is attending|{2} are attending|]2,Inf] and %count% - 2 others are attending
{% endtranschoice %}

Thanks in advance.

translation symfony twig
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